Brightness limiter for image intensifiers

ABSTRACT

The invention is directed to a system for automatically limiting the output brightness of an image intensifier having a microchannel plate (MCP). The intensifier includes a radiationsensitive surface which is located in the proximity of the input surface of the MCP. An output screen is located in the proximity of the output surface of the MCP. The two surfaces of the MCP are maintained at different voltages which accelerate electrons through the MCP to cause the multiplication of electrons therein. The output brightness of the intensifier is directly proportional to the current supplied to the output screen. This current is dependent upon the voltage supplied across the microchannel input and output surfaces. Therefore, the system senses the output screen current and changes the voltage applied across the MCP to limit the sensed current and the output brightness to some desired level.

United States Patent Wyess [451 May 30, 1972 [54] BRIGHTNESS LIMITER FORIMAGE INTENSIFIERS William R. Wyess, Livonia, Mich.

[73] Assignee: The Bendix Corporation [22] Filed: Jan. 25, 1971 [21]Appl. No.: 109,413

[72] Inventor:

Primary Examiner-John S. Heyman Assistant Examiner-R. E. HartAttorney-Lester L. Hallacher and Flame, Hartz, Smith and ThompsonABSTRACT The invention is directed to a system for automaticallylimiting the output brightness of an image intensifier having amicrochannel plate (MCP). The intensifier includes a radiation-sensitive surface which is located in the proximity of the inputsurface of the MCP. An output screen is located in the proximity of theoutput surface of the MCP. The two surfaces of the MC? are maintained atdifferent voltages which accelerate electrons through the MCP to causethe multiplication of electrons therein. The output brightness of theintensifier is directly proportional to the current supplied to theoutput screen. This current is dependent upon the voltage suppliedacross the microchannel input and output surfaces. Therefore, the systemsenses the output screen current and changes the voltage applied acrossthe MC? to limit the sensed current and the output brightness to somedesired level.

7 Claims, 2 Drawing Figures Patented May 30, 1972 QQR 1 QPQYNDQ INVENTORWILLIAM R. Wyess ATTORNEY BRlGI-ITNESS LIMITER FOR IMAGE INTENSIFIERSBACKGROUND OF THE INVENTION Image intensifiers utilizing microchannelplates (MCP) are well known in the art. In such intensifiers amicrochannel plate is placed between a radiation-sensitive input screenand an output screen which yields a visible output. The output screentherefore is frequently a phosphorous screen. Radiation, such as photonsor charged particles, incident on the input screen cause the release ofelectrons from the input surface. The electrons pass to the MCP wherethey are multiplied by several orders of magnitude because of secondaryemission effects in the MCP. The multiplied electrons emanate from theMCP and impact with the output screen so that each electron causes thescreen'to fluoresce. Because of the structure of the MCP, any imageinformation carried by the radiation impinging on the input surface ispreserved and reproduced on the output surface as a highly intensifiedimage.

intensification of the input image is highly desirable and is theprimary purpose of the MCP. However, overintensification can bedetrimental. If the brightness of the output image is excessive,contrast and image sharpness are lost, resulting in a reduction of theusefulness of the output image. Moreover, the life of the imageintensifier could be impaired.

SUMMARY OF THE INVENTION The invention is directed to a system forlimiting the brightness of an image intensifier of the type describedhereinabove. Some of the disadvantages of such intensifiers aretherefore overcome by the inventive system.

The output brightness of an image intensifier is directly proportionalto the current supplied to the output screen of the intensifier tube.This current is caused by the flow of electrons from the MCP to theoutput screen and, for a given incident radiation on the input screen,is dependent upon the biasing voltage supplied across the MCP.Accordingly, the invention is directed to a system which limits thecurrent in the output screen.

Current limiting is achieved by utilizing a current-sensitive biasingsource to supply a voltage across the MCP. The-output screen current issensed and used to control the output of the biasing source and thuslimit the brightness of the intensifier. 1

The input and output surfaces of the MCP are respectively located in theproximity of a radiation-sensitive input screen and an output screencapable of sustaining a visual output image. The input and output screenare maintained at different constant voltages so that electrons travelfrom the input screen to the output screen. A current-sensitive voltagemeans, such as a current controlled oscillator, biases the outputsurface of the MCP. Accordingly, the intensity of the output image islimited to a level insuring a high contrast and sharp image.

BRIEF DESCRIPTION OF THE DRAWINGS Figure l is a preferred embodiment ofthe inventive system. Figure 2 is a preferred embodiment of acurrent-sensitive biasing source which can be used in the inventivesystem.

DETAILED DESCRIPTION Figure 1 shows a Wafer'lmage Intensifier which isused to recieve and intensify a radiation image. The Image Intensifier10 includes three active elements: Microchannel Plate (MC P) 11;Photocathode 12, which is the radiation-sensitive input screen; andPhosphor Screen 16, which is the output screen of the intensifier tube.Photocathode 12 is placed in the close proximity of the Input Surface 13of MCP l1, and

Phosphor Output Screen 16 is placed in the close proximity.

of Output Surface 14 of MCP 11. Input Screen 12 and Output Screen 16 areplaced near the Input and Output Surfaces l3 and 14, respectively, ofMCP 11 so that a current How is created by electron exchange between thesurfaces and the MCP 11. However, the screens are positioned withrespectto the Input and Output Surfaces such that they willsupport differentbiasing potentials with respect to the potentials ofthe MCP surfaces.

In operation, a radiation image impinging upon input screen orPhotocathode 12 causes the emission of electrons by Photocathode 12. Anyimage information contained within the impinging radiation is reproducedin the form of a corresponding electron image because of photoemissionby the Photocathode 12. The electrons, and therefore the electron image,are attracted to Input Surface 13 of MCP 11. This is so because InputSurface 13 of MCP 1 l is maintained at a higher positive potential thanPhotocathode Screen 12. Accordingly,

electrons emanating from Photocathode 12 are attracted to the InputSurface 13 of MCP 1 1. 3

Microchannel Plate 11 consists of a bundle of discrete hollow glasstubes, or channels, which are capable of amplifying an electron image bymany orders of magnitude. Each electron impinging upon Input Surface 13of MCP 1 1 results in the emission of a number of secondary electronswithin the MCP 11. The secondary electrons then cause the emission ofmore secondary electrons so that the number of electrons emanating fromOutput Surface 14 of MCP 1 1 is substantially higher, by the order ofmany magnitudes, than the number of electrons impinging on 'lnputSurface13. The electron gain or multiplication within the MCP is controlledprimarily by the potential difference applied across the Input andOutput Surfaces 13 and 14, respectively, of the MCP. Because of theelectron multiplication which occurs within the MCP, the radiationimpinging upon Photocathode 12 results in the emanation of a quantity ofelectrons from Output Surface 13 which greatly exceeds the quantityinitially produced by the radiation. Image information is preserved bythe close proximity of Screen 12 to the surface 13 of the MCP and by thehigh density of discrete hollow channels of which the MCP is comprised.And also because the electron multiplication occasioned by a singleelectron occurs in only one channel. Accordingly, the point-to-pointvariations of the image of the impacting radiation are preserved. Theinput image information is therefore reproduced in a highly intensifiedform at Output Surface 14 of MCP 11. The electrons emanating from MCP 11and containing the input radiation image information impinge withPhosphorous Output Screen 16, causing the screen to fluoresce andreproduce the input image. The close proximity of Screen 16 to OutputSurface 14 also is instrumental in preserving the image information.

' Because electrons are always attracted through the image intensifiertube from Photocathode 13 to Phosphorous Output Screen 16, the voltagesapplied to the various surfaces and screens become more positive in thedirection of electron flow.

A potential is applied to Photocathode 12 by a constant voltage ACSource 17 through a Multiplier-Rectifier l8. Multiplier-Rectifier 18rectifies the AC signal so that a constant DC potential is applied toPhotocathode 12. If desired, this can be applied through a CurrentLimiting Resistor 26. Multiplier-Rectifier 18 includes a Capacitor 19which is connected to the oppositely poled Diodes 21 and 22. The anodeof Diode 21 is connected to the cathode of Diode 22 through a Capacitor23 at Junction 24. This circuit operates in known manner to double thezero to peak input voltage and also to rectify the voltage so that aconstant DC voltage is obtained at the output thereof. Diodes 21 and 22conduct on alternate half cycles of AC Source 17 according to theirpolarity. Capacitor 23 is thereby charged so that the voltage output ofMultiplier 18 with reference to ground is equivalent to the peak ACinput voltage of AC Source 17 plus the voltage present at Junction 24.AC Source 17 can be an output of a regulated oscillator so that the DCpotential applied to Photocathode 12 is constant.

The biasing voltage across Input Surface 13 and Output Surface 14 of MCPl l is applied from a Variable Voltage Oscillator 29 through threeMultiplying Sections 26, 27 and 28 which are constructed and operate inthe same manner as Multiplierage. The DC voltage across Capacitor 32 isapplied across Surfaces 13 and 14 of MCP 11. Obviously, if desired, moreor less sections can be used to increase or decrease the potentialacross MCP 1 l. I

It will be noted that Multiplier-Rectifier 18 is referenced to thevoltage applied to Surface 13 of MCP 11 at Junction 24. Accordingly, thebiasing voltage applied to Photocathode l2 varies by the same amount asthe voltage applied to Input Surface 13. This is done in order tomaintain a constant voltage differential between Photocathode l2 and MCPlnput Surface 13.

Phosphorous Output Screen 16 is biased by an AC Source 33 which isrectified and multiplied through Multiplying Section 34. MultiplyingSection 34 will contain the desired number of multiplying sections sothat the DC output voltage is substantially multiplied from the peak ACinput voltage supplied by AC Source 33, as shown by the partialsectioning of the multiplyingsection.

The path of Screen Current l, is defined by the electron flow betweenOutput Surface 14 and Phosphorous Screen 16, Lead 25, Current SensingResistor 36, Multiplying Section 34, and Line 37, which connects themultiplier section to Phosphorous Screen 16. The output intensity of theintensifier, as evidenced by the visual image on Phosphorous Screen 16,is dependent upon the DC voltage applied across MCP 11 from AC Source29. Current I, is dependent upon this voltage, and accordingly theoutputimage intensity can be limited by sensing the Current I, and usingthis current to control. the voltage amplitude of Oscillator 29. ScreenCurrent Sensing Resistor 36, which is bypassed by a smoothing Capacitor37, is used to sense the screen current and control the voltage ofOscillator 29. a

The manner in which Current 1, controls Oscillator 29 is illustratedwith reference to FIG. 2, which shows an exemplary embodiment of anoscillator which can be used to control the biasing voltage. appliedacross MCP 11. FIG. 2 also shows another Oscillator 17 which can be usedto supply the constant AC voltages, AC and AC to Photocathode 12 andPhosphorous Screen 16.

As shown in FIG. 2, Oscillator 29 includes a regulator section comprisedof Transistors Q through Q and an oscillator section comprised ofTransistors Q and Q-,. As shown in FIGS. 1 and 2, Junction 38 ofResistor 36 and Capacitor 37 is connected to the base of Transistor QWhen the current through Current Sensing Resistor 36 is below the levelat which limiting is to occur, Transistor O is cutoff. For thiscondition, O is saturated and the regulator-oscillator circuit operatesin a voltage regulation mode to provide a constant voltage to MCP 11.This is accomplished by maintaining a constant transformer feedbackvoltage at Junction 45. The voltage at Junction 45 is compared to thereference voltage developed by Zener Diode 43 and the base to emitterpotential of Q If a potential difference occurs, the error signal isamplified by Transistors Q Q and Q and applied as a DC level change tothe input of Oscillator Q and Q, at Junction 46. Accordingly, thefeedback voltage at Junction 45 is automatically readjusted to reducethe error signal to a near-zero level. The output of the oscillator isthus maintained at a near constant level. The biasing voltage AC 2,which sets the amplitude of the DC voltage supplied across MCP 11, istaken off the Secondary 42 of Transformer 41. When Oscillator 29 isoperated in a voltage regulation mode, its output amplitude can bemanually controlled through Potentiometer 44.

When the current through Current Sensing Resistor 36 rises to itsprescribed limiting level, the voltage across Resistor 36 rises abovethe threshold conduction level of O For this condition, Oscillator 29switches over to a current regulator mode to provide a constant screencurrent characteristic. When Q conducts as a result of the screemcurrentreaching its prescribed limiting level,.the conduction of Q, is reduced.

This, in turn, reduces the conduction level of 0. thereby lowering theinput voltage of Oscillator Q, and Q, at Junction 46. As a result, theamplitude of the output of Oscillator 29 is reduced proportionally andthe biasing potential applied across MCP 11 is lowered to maintainthe'sensed screen current at its prescribed limiting level. Attendentlythe intensity or brightness of the output image present on PhosphorousScreen 16 is limited also.

Oscillator 17, shown in FIG. 2, can be of any of several known types. Asan example, if Transistors Q1, Q and Resistor 41 of regulated Oscillator29 are eliminated so that the emitter of Transistor O is groundedthrough Resistor 46, this oscillator can be used as Oscillator 17.

What is claimed is:

1. A system for limiting the intensity of the output image of amicrochannel plate intensifier comprising:

a microchannel plate having an input surface and an output surface; aradiation sensitive input screen in the proximity of said input surface,and an output screen capable of sustaining an output image in theproximity of said output surface so that image information present ininput radiation impinging upon said input screen is intensified anddiscernable on said output screen;

means for maintaining said input screen at a first constant voltage;

means for maintaining said output screen at a second constant voltage;

current sensitive voltage means for applying a varying voltage acrosssaid microchannel plate, said current sensitive voltage means includinga voltage controlled oscillator and means for changing the voltageamplitude of the output of said oscillator resistive means in serieswith said output screen for sensing current flow to said output screen,said current sensitive voltage means being responsive to said means forsensing to change said variable voltage in response to said outputscreen current.

2. The system of claim 1 wherein said means for changing 7 includeselectron control means responsive to said output screen current. I

3. The system of claim 2 wherein said electron control means is atransistor, the control electrode of said transistor being coupled tosaid resistor.

4. The system of claim 3 wherein said transistor decreases saidoscillator output voltage when said screen current is sufficient torender said transistor conductive.

5. The system of claim 4 further including first multiplierrectifiermeans interposed between said oscillator and said input surface so thata varying DC voltage is applied across said microchannel plate.

6. The system of claim 5 wherein said means for maintaining said inputscreen includes a second multiplier-rectifier interposed between saidinput screen and an AC supply;

and said means for maintaining said output screen includes a thirdmultiplier-rectified interposed between said output screen and an ACsupply;

said constant voltages being DC voltages having a level at least as highas the peak voltages of the respective AC supply.

7. The system of claim 3 wherein said transistor is a field effecttransistor.

1. A system for limiting the intensity of the output image of amicrochannel plate intensifier comprising: a microchannel plate havingan input surface and an output surface; a radiation sensitive inputscreen in the proximity of said input surface, and an output screencapable of sustaining an output image in the proximity of said outputsurface so that image information present in input radiation impingingupon said input screen is intensified and discernable on said outputscreen; means for maintaining said input screen at a first constantvoltage; means for maintaining said output screen at a second constantvoltage; current sensitive voltage means for applying a varying voltageacross said microchannel plate, said current sensiTive voltage meansincluding a voltage controlled oscillator and means for changing thevoltage amplitude of the output of said oscillator resistive means inseries with said output screen for sensing current flow to said outputscreen, said current sensitive voltage means being responsive to saidmeans for sensing to change said variable voltage in response to saidoutput screen current.
 2. The system of claim 1 wherein said means forchanging includes electron control means responsive to said outputscreen current.
 3. The system of claim 2 wherein said electron controlmeans is a transistor, the control electrode of said transistor beingcoupled to said resistor.
 4. The system of claim 3 wherein saidtransistor decreases said oscillator output voltage when said screencurrent is sufficient to render said transistor conductive.
 5. Thesystem of claim 4 further including first multiplier-rectifier meansinterposed between said oscillator and said input surface so that avarying DC voltage is applied across said microchannel plate.
 6. Thesystem of claim 5 wherein said means for maintaining said input screenincludes a second multiplier-rectifier interposed between said inputscreen and an AC supply; and said means for maintaining said outputscreen includes a third multiplier-rectified interposed between saidoutput screen and an AC supply; said constant voltages being DC voltageshaving a level at least as high as the peak voltages of the respectiveAC supply.
 7. The system of claim 3 wherein said transistor is a fieldeffect transistor.